ensure file is not in cache before calling require

[finnigantime]

Description of the Change

Fixes #3084.

Alternate Designs

Could have modified require.cache directly, but seems easier to add the decache dependency and it handle modifying the require.cache for us.

Why should this be in core?

Mocha makes an assumption that when loading files, require(file) will parse a file and encounter mocha directives. For this to be valid, mocha should ensure the file is not in the require cache.

Benefits

Allows mocha to be used programmatically in scripts/environments where modules mocha is going to run tests on have already been loaded (perhaps by a previous invocation of mocha itself).

Possible Drawbacks

None that come to mind immediately, other than risk by having the decache dependency, though I think having that dependency makes sense here.

Applicable issues

bug fix

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.05%) to 89.86% when pulling 8618e7f on finnigantime:finnigantime/decache into da901da on mochajs:master.

[coveralls]

Coverage increased (+0.05%) to 89.86% when pulling 7c3e89d on finnigantime:finnigantime/decache into da901da on mochajs:master.

[boneskull]

thanks. as @ScottFreeCode wrote in #3084, we can't just blast the cache wholesale. if we're going to have this capability baked in to core, the solution is going to be much more nuanced.